HELICOPDES PRELIMS - Review

Propeller

Rotating airfoil that converts engine power into thrust. Consists of hub + helical blades (converts shaft power into aerodynamic thrust).

Tractor (Front-mounted)

Most common, operates in cleaner air; mounted on the upstream end of a drive shaft in front of the supporting structure; advantage: lower stresses induced, as it rotates in relatively undisturbed air.

Pusher (Rear-mounted)

Mounted on the downstream end of a drive shaft behind the supporting structure; disadvantage: subject to more damage than tractor propellers.

Hub

Center portion of propeller.

Hub bore

Hole for crankshaft or reduction-gear assembly connection.

Blade root/shank

Nearest to hub.

Blade tip

Farthest from hub.

Blade face

Lower surface or flat side.

Blade back/thrust face

Curved surface.

Leading edge

Front cutting edge that slices into air.

Trailing edge

Air-exit edge.

Blade cuff

Airfoil-shaped attachment near root.

Chord line

Imaginary straight line from leading to trailing edge, aligned with normal airflow.

Blade angle (blade pitch)

Angle between chord line and plane of rotation.

Plane of rotation

Circular plane in which the blades rotate.

Axis of rotation

Fixed axis around which rotation occurs.

Angle of attack (AOA)

Angle formed by the chord line and the relative wind.

Blade station

Distance from the center of the hub.

Blade twist

Change in chord line from root to tip for equal lift distribution.

Propeller Blade Element Theory

The variation in airfoil shape and blade angle along the length of a propeller blade compensates for differences in rotational speed and allows for a more even distribution of thrust along the blade.

Geometric pitch

Ideal forward distance traveled per one revolution.

Effective pitch

Actual forward distance traveled per one revolution.

Propeller slip

Difference between geometric pitch and effective pitch.

Wood

1) Wood Selection: free of grain irregularities, knots, pitch pockets, insect damage; 2) Lamination: min. five layers kiln-dried, waterproof glue; 3) Shaping & Drilling: blade shaped, hub drilled; 4) Surface Reinforcement: fabric sheathing, varnish; 5) Metal Tipping: scalloped/plain on tips & leading edges to prevent erosion; 6) Breathing Holes: three 3/16″ holes per tip to release moisture.

Aluminum Alloy

1) High-strength, heat-treated alloy forged from single bar; 2) Shaping: machine & manual grinding; 3) Pitch Setting: blades twisted to final pitch; 4) Stress Relief: heat treatment to relieve internal stresses.

Steel

Rare, heavy, mostly hollow with foam fill; constructed from steel sheets attached to rib structure.

Composite

Light, extremely durable, corrosion-resistant; made of fiber and matrix constituents.

Fixed-Pitch

One-piece, non-adjustable; types: Standard, Climb, Cruise.

Ground-Adjustable

Blade angle adjusted on ground within preset fine/coarse limits.

Two-Position

Pilot selects between two blade angles in flight (takeoff vs. cruise).

Controllable-Pitch

Pilot manually adjusts blade angle anytime in flight.

Constant-Speed

Governor automatically adjusts blade pitch to maintain constant RPM.

Propeller Governor

Monitors engine RPM and adjusts blade angle automatically to hold chosen RPM.

Push Lever Forward

Increases speeder spring tension → oil escapes hub, piston forward → blade angle decreases → RPM increases; flyweights fall in, lowering valve, allowing oil to sump; as RPM rises, flyweights return, stopping flow.

Pull Lever Back

Releases speeder spring tension → oil forces piston back → blade angle increases → RPM decreases; flyweights fling out, lifting valve, pumping oil into hub; as RPM falls, flyweights return, stopping flow.

On-Speed

RPM equals pilot's set value; flyweights & spring in balance; no oil flow.

Under-Speed

RPM too low; spring dominates → oil flow increases → blades to low pitch → RPM rises.

Over-Speed

RPM too high; centrifugal force dominates → oil flow decreases → blades to high pitch → RPM falls.

Full Feathering

Blades rotate to edge-into-wind to stop rotation and reduce drag (used in engine failure).

Reverse Pitch

Negative blade angle; produces reverse thrust (used to slow aircraft).

Alpha Range

Idle to max power during normal flight (takeoff to landing); governor holds RPM; power lever controls fuel only.

Beta Range

From flight idle to max reverse during ground ops/braking; below flight idle power levers directly control pitch; ground idle = min thrust; advancing lever into reverse produces reverse pitch.

Thrust Force

Pulls aircraft forward; bends blades forward.

Centrifugal Force

Pulls blades outward due to rotation.

Aerodynamic Twisting Force

Twists blades toward low blade angle (flattening tendency).

Lift & Drag

Lift = forward thrust; drag = resists motion; includes induced & parasite drag.

Static Unbalance

CG not on axis of rotation.

Dynamic Unbalance

CGs of components not in same plane.

Aerodynamic Unbalance

Unequal blade thrust (check contour).

Torque Effect

Right-turning prop pushes left side of aircraft down → increased left tire friction → yaw left.

P-Factor (Asymmetric Loading)

At high AOA/downward-moving blade takes bigger bite → more thrust → yaw left (common in tailwheel).

Gyroscopic Precession

Spinning prop acts as gyroscope; force felt 90° ahead in rotation → yaw left.

Spiraling Slipstream

Prop wash spirals rearward, strikes vertical tail → sideward force → yaw left.